Physiology, Plasma Osmolality and Oncotic Pressure

Excerpt

The plasma osmolality and oncotic pressures in an organism can determine the direction of fluid movement within the system; therefore, the relative concentration of ions and protein in the solvent. As a result, we can observe the fluid movement results, which can typically manifest as edema, dehydration, changes in blood pressure, seizures, and changes in intracranial pressure. Furthermore, osmolality disturbances can be used as an indication for the use of intravenous fluids, which can be used to quickly alter the plasma osmolality and oncotic pressures in the vascular system.

Osmolarity is the number of milliosmoles of solute per liter solution. This is different from osmolality (osm), which is the milliosmoles of solute per kilogram of solution.

Water flows from a compartment of low osmolality to a compartment with high osmolality. This can only occur if the membrane between the two compartments is permeable to water. An example of this is when comparing plasma osm and interstitial fluid osm. At the cellular level, we can compare the intracellular osm to the extracellular matrix. In this system, the phospholipid bilayer serves as the semipermeable membrane through which water can flow.